The granulation of potassium fertilizers of potassium chloride via press-granulation is a worldwide preferred procedure in order to provide granules with grain sizes between 1 to 5 mm from fine grained crystallizates or flotation concentrates. These granules can be spread mechanically and provide the desired spread pattern when spilled.
The process of press-granulating of potassium chloride and other chloride-containing salts generally functions without granulate auxiliaries, i.e. is free of binders. In the press-granulating process, the dried hot fine salt together with salt return (undersized grain) is provided to a roller press via a stuffing screw, and is compacted via counter-rotating, usually profiled, rollers under high pressure to a foil consisting of compacted salt (scraps). The foil exiting the roller press is separated from the non-pressed salt and is crushed in an impact mill or a hammer mill. The crushed material is separated using an oscillating sieve into coarse grain (oversized material), product (medium grain) and fine product (undersized grain). Coarse grain is further crushed, and undersized grain together with fresh salt is provided to the roller press again. Said process results in the desired product grain size, preferably from 1 to 4 or from 2 to 5 mm.
Said process has been successfully used for chloride-containing salts, but does not work for salts which are difficult to press, especially sulfate-containing salts, since the compacting using a roller press requires a certain ductility and fluidity of the salts under high pressure, which is true for potassium chloride and sodium chloride, but not for the mentioned potassium-, magnesium- and calcium containing sulfate minerals. Only pure potassium sulfate can be granuled using press-granulation under specific circumstances (such as addition of some water or blasting of vapor).
Non-ductile salts, such as the above-mentioned sulfate minerals, are generally processed to granulate by roll granulation using pan granulators or granulation drums, wherein generally granulate auxiliaries and additives have to be added. Roll granulates are also usable spillable products, but show a generally different spillage-, mixture- and storage behavior as compared to press granulates, which has to be taken into account when mixing different single fertilizers (bulk blending). Furthermore, roll granulates require an additional thermal drying treatment.